Conventional polymethylmethacrylate bone cements (PMMA bone cements) have been known for decades and are based on the ground-breaking work of Sir Charnley (Charnley, J.: “Anchorage of the femoral head prosthesis of the shaft of the femur”; J. Bone Joint Surg. 42 (1960) 28-30). The basic structure of PMMA bone cements has remained the same ever since. PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains (i) the monomer, methylmethacrylate, and (ii) an activator (e.g. N,N-dimethyl-p-toluidine) dissolved therein. The powder component comprises (i) one or more polymers that are made by polymerisation, preferably by suspension polymerisation, based on methylmethacrylate and co-monomers, such as styrene, methylacrylate or similar monomers, (ii) a radio-opaquer, and (iii) an initiator, (e.g. dibenzoylperoxide). Mixing the powder component and the monomer component, the polymers of the powder component in the methylmethacrylate swell which generates a dough that can be shaped plastically. Simultaneously, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide which disintegrates and forms radicals in the process. The radicals thus formed trigger the radical polymerisation of the methylmethacrylate. Upon advancing polymerisation of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies and thus is cured.
The essential disadvantage of the previous PMMA bone cements for the medical user is that the user needs to mix the liquid monomer component and the powder component in a mixing system or in crucibles right before applying the cement. Mixing errors can easily occur in the process and adversely affect the quality of the cement. Moreover, the components must be mixed rapidly. In this context, it is important to mix all of the cement powder and monomer component without forming lumps and prevent the introduction of air bubbles during the mixing process. Unlike manual mixing, the use of vacuum mixing systems prevents the formation of air bubbles in the cement dough to a large extent. Examples of mixing systems are disclosed in patent specifications U.S. Pat. No. 4,015,945, EP-A-0 674 888, and JP-A-2003181270. However, vacuum mixing systems necessitate an additional vacuum pump and are therefore relatively expensive. Moreover, depending on the type of cement concerned, a certain waiting time is required after mixing the monomer component and the powder component until the cement dough is tack-free and can be applied. Because of the large variety of errors that can occur while mixing conventional PMMA bone cements, appropriately trained personnel is required for this purpose. The corresponding training is associated with considerable expenses. Moreover, mixing of the liquid monomer component and the powder component is associated with exposure of the user to monomer vapours and particles released from the powder-like cement.
Pasty polymethylmethacrylate bone cements containing a methacrylate monomer for radical polymerisation, a polymer that is soluble in said methacrylate monomer, and a particulate polymer that is insoluble in said methacrylate monomer have been described as an alternative to the conventional powder-liquid polymethylmethacrylate bone cements in unexamined German patent applications DE-A-10 2007 052 116, DE-A-10 2007 050 762, and DE-A-10 2007 050 763. Paste-like polymethylmethacrylate bone cements of this type can be present as one-component systems (in this case, the paste contains all components required for curing, in particular an activatable radical initiator, e.g. a photoinitiator or a photoinitiator system) or as two-component systems (in this case, the system comprises two pre-mixed pastes that are stable on storage and one of which comprises a radical polymerisation initiator and the other comprises a polymerisation activator). Referring to two-component systems, a distinction is made between a “symmetrical system” (in this case both pastes contain a particulate polymer that is insoluble in the methacrylate monomer) and “non-symmetrical systems” (in this case, only one of the two pastes contains a particulate polymer that is insoluble in the methacrylate monomer).
As a result of the selected composition, the bone cement produced from the pastes described above possesses sufficiently high viscosity and cohesion in order to withstand the pressure from bleeding until it is fully cured. Owing to the advancing polymerisation, the paste is cured while the methacrylate monomers are consumed.
Aside from at least one monomer for radical polymerisation and at least one polymer dissolved therein, the pasty polymethylmethacrylate bone cements disclosed in DE-A-10 2007 052 116, DE-A-10 2007 050 762, and DE-A-10 2007 050 763 contain polymer particles that are insoluble in said monomer. Said insoluble polymer particles are a filling agent. Said filling agent has a significant influence on the viscosity of the cement pastes. The polymer particles are essential for the processing properties to ensure that the cement pastes show as little restoring motion as possible during the application phase of the shaping process. This allows the cement pastes to be moulded into any shape during the processing phase such as is generally known for conventional polymethylmethacrylate bone cements that are based on the mixing of polymer powder and monomer liquid.
The production of cross-linked polymer particles that are insoluble in methacrylate monomers is relatively laborious and therefore expensive. For this reason, it is desirable to identify an alternative, inexpensive particulate material which, after admixture into mixtures of methacrylate monomers and polymers dissolved therein, yields pastes that show only minimal elastic resilience after shaping much like cross-linked polymer particles.
However, one problem is that the cross-linked polymer particles used thus far also contributed to the mechanical stability of the cured pasty cements. It is therefore important to identify an alternative filling agent which not only ensures that the pastes have the requisite processing properties, but also does not adversely affect the mechanical parameters of the cured cements such that the mechanical stability requirements of ISO 5833 are met.